A priori: DFW is a fortress hub and upgrades through there tend to be tough in general. LA is a larger city and LAX has more competition (especially to Japan) and LAX-Japan is a shorter route so the upgrade is slightly less desirable.

Empirically: Last summer I planned to burn 4x SWUs to or from Japan and searched +/- a few weeks of any C availability. LAX-HND turned up a few dates, but I seem to recall DFW had nothing.

Also, though less relevant: I used to travel US-Asia 5-6x/yr in the 00s and 10s. LAX and ORD usually cleared or I could find space in advance (oh, those were the days). DFW rarely had space ahead of time and I missed a few.